Microwave filter comprising a coaxial structure with a metallized foam having a periodic profile

ABSTRACT

The invention relates to a microwave filter comprising a coaxial structure, consisting of a tubular outer conductor and an inner bar conductor. According to the invention, the inner bar conductor extends in an axial direction inside the outer tube and, together with said tube, forms a series of concentric slots in the axial direction thereby defining successive coaxial line segments with low characteristic impedance and coaxial line segments with high characteristic impedance. The aforementioned concentric slots are produced in a synthetic foam block.

This application claims the benefit, under 35 U.S.C. § 365 ofInternational Application PCT/FR03/50200, filed Dec. 22, 2003, which waspublished in accordance with PCT Article 21(2) on Aug. 5, 2004 in Frenchand which claims the benefit of French patent application No. 0300048,filed Jan. 3, 2003.

The invention relates to a coaxial structure microwave filter comprisingan outer conductive core and an inner conductive core extendingaccording to an axial direction within the outer core and forming withthis core a succession of concentric crenelations according to an axialdirection defining successive sections of low characteristic impedancecoaxial lines and high characteristic impedance coaxial lines.

BACKGROUND OF THE INVENTION

The work “Microwave Filters, Impedance-Matching Networks and CouplingStructures”, McGraw-Hill, 1962, describes such a microwave filter, inparticular a low-pass filter, in which the outer conductive core isnormally constituted by a cylindrical metal rod carrying concentricmetal disks spaced according to the axial direction, the metal disksforming the succession of concentric crenelations. The cross-section ofthe inner core thus varies according to the axial direction so that eachsection of the large diameter inner core (corresponding to a metal disk)defines a section of coaxial line of very low impedance and each sectionof inner core of smaller diameter (corresponding to the interval betweentwo consecutive disks) defines a section of coaxial line of highimpedance. The dimensions of the sections are adjusted so as to realizethe transfer function of the filter. However, the realization of such acoaxial structure microwave filter proves to be complex and costly,particularly for maintaining the inner core and the outer core of thefilter perfectly coaxial. Spacers made from plastic or anotherdielectric material are generally used to maintain them coaxial but thisintroduces dielectric losses.

SUMMARY OF THE INVENTION

The invention proposes a coaxial structure microwave filter of a simplerand less expensive construction suitable for low cost volume production.

For this purpose, the invention relates to a coaxial structure microwavefilter constituted by a tube of synthetic foam material, the tubepresenting a constant internal diameter and a fully metallized externalsurface with, in the axial direction, a profile according to a periodicor constant function and by a bar of a fully metallized syntheticmaterial, with a constant external profile or following a periodicfunction, the largest diameter of the bar being noticeably equal to theinternal diameter of the tube so that the bar can be inserted into thetube while maintaining the tube and the bar coaxial. The foam used ispreferably a polymethacrylimide foam known for its electricalcharacteristics approaching those of air, for its mechanicalcharacteristics of rigidity and lightness and for its low cost price. Inparticular, a polymethacrylimide foam under the name ofpolymethacrylimide HF (high frequency) can be used.

According to the particularities of a filter according to the invention:

-   -   The periodic or constant function per part depends on        crenelations, the crenelations being able to have dimensions        that differ from one crenelation to another.    -   The thickness of the tube is chosen to maintain electrical        insulation between the metallized surface of the tube and the        bar

With this construction, a microwave filter can easily be combined with amonopole type or dipole type antenna.

The invention extends to a method of producing a microwave filter asdefined above according to which the periodic function is realized bythermoforming the foam tube or foam bar. In particular, as athermoforming technique, hot press molding will preferably be used,which is adapted to an objective of high volume, low cost production.

The metallization of the foam tube or foam bar is preferably anon-directive metallization by projection or brush.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of a filter according to the invention are described belowand illustrated in the drawings.

FIG. 1 shows an exploded perspective, in a highly schematic manner, of afirst embodiment of a coaxial structure microwave filter according tothe invention.

FIG. 2 schematically shows an axial section of a second embodiment of acoaxial structure microwave filter according to the invention associatedwith a monopole type antenna.

FIG. 3 schematically shows an axial section of a filter according to thefirst embodiment associated with a dipole type antenna.

DESCRIPTION OF PREFERRED EMBODIMENTS

A first example of a coaxial structure microwave filter according to theinvention is shown in FIG. 1 according to an exploded perspective view.

The outer conductive tube 1 and the inner conductive bar 2 of the filterare shown in FIG. 1 dissociated from each other for greater clarity, butit must be understood that the inner bar 2 extends according to theaxial direction A inside the outer tube 1.

The inner bar 2 of the filter is constituted by a cylindrical bar madeof synthetic foam whose outer surface follows a periodic functionaccording to the axial direction. It preferably forms a succession ofconcentric crenelations 3A, 3B, 3C and 3D realizing the transferfunction of the filter, for example a transfer function of a low-passfilter by defining successive sections of low characteristic impedancecoaxial lines and high characteristic impedance coaxial lines. The shapeof the foam bar 2 is realized by thermoforming, in particular accordingto a hot press molding technique. The outer surface is metallized byusing a step of metal projecting or of metallic brushing (painting).

The outer tube 1 of the filter is constituted by a cylindrical tube ofsynthetic foam having a constant inner cross-section, the inner diameterof the tube being very slightly greater at the largest outer diameter ofthe foam bar 2 to allow the bar to be inserted into the tube. Thecylindrical tube 1 has an outer surface fully metallized according tothe technique described above. The thickness of the tube 1 is chosen torealize an electrical insulation between its outer metallized surfaceand the bar

The synthetic material foam used is preferably a polymethacrylate imidefoam.

The structure of the filter shown in FIG. 1 can be reinforced by twohalf-shells (not shown) surrounding the tube 1 that can be realized in aplastic material or in synthetic foam material.

Naturally, the tube 1 and foam bar 2 can have a cross-section other thancircular, for example rectangular or square without falling outside thescope of the invention.

FIG. 2 shows another embodiment of a filter according to the invention.The outer tube 1′ of the filter is constituted by a constituted by acylindrical tube of synthetic foam material whose outer metallizedsurface is conformed to define the succession of crenelations 3A′-3B′according to the axial direction A whereas the inner bar 2′ of thefilter is constituted by a conductive cylindrical bar of constantcross-section. In this manner, the outer surface of the tube presents,according to the axial direction, a profile following a periodic orconstant function by parts such as a crenelation function. Theconductive bar 2′ can consist of a solid or hollow cylindrical metaltube. The bar 2′ can also be constituted by metallized syntheticmaterial foam. In FIG. 2, the microwave filter according to theinvention is associated with a monopole type antenna 4 constituted by anextension of the inner core 2′ of the filter.

FIG. 3 shows a microwave filter according to the invention that issimilar to the filter shown in FIG. 1 with an outer foam tube 1″ ofconstant cross-section and an inner bar constituted by a foam bar 2″ ofvariable cross-section according to the axial direction A. Here, thefilter is associated with a dipole type antenna 5.

The use of the metallized foam technique enables complex coaxialstructure microwave filters to be realized at low cost.

1. A coaxial structure microwave filter comprising a tube presenting aconstant inner diameter and a fully metallized constant outer surfaceand an inner bar with a fully metallized outer profile following aperiodic curve, the tube and the bar being realized in foam of ametallizable synthetic material with electrical characteristicsapproaching those of air, the inner bar having a largest diameter nearlyequal to the inner diameter of the tube.
 2. The filter according toclaim 1, wherein the periodic curve includes a plurality ofcrenelations, the crenelations having dimensions identical from onecrenelation to another.
 3. The filter according to claim 1, wherein theperiodic curve includes a plurality of crenelations, the crenelationshaving dimensions different from one crenelation to another.
 4. Acoaxial structure microwave filter comprising a tube presenting aconstant internal diameter and a fully metallized outer surface with, inan axial direction, a profile following a periodic curve and an innerbar with a constant fully metallized outer profile, the tube and the barbeing realized in foam of a metallizable synthetic material withelectrical characteristics approaching those of air, a largest diameterof the bar being noticeably equal to the internal diameter of the tube.5. The filter according to claim 4, wherein the periodic curve includesa plurality of crenelations, the crenelations having dimensionsidentical from one crenelation to another.
 6. The filter according toclaim 4, wherein the periodic curve includes a plurality ofcrenelations, the crenelations having dimensions different from onecrenelation to another.